Existing disturbances and short-time fluctuations have a great influence on practical measurements in many linear time-invariant systems. They limit the possible quality of the result. This is the case, in particular, when measurements are taken with spectrally limited or time-limited and interrupted excitation signals, such as with speech or music signals in audio engineering and acoustics.
A variety of processes are known which have been proposed in order to improve measuring results:                taking the average across several measurements, which increases the signal-to-noise-ratio by 3 dB for each doubling of the number of measurements if the interference is not correlated. The user can choose whether to take a complex or power average. This may result, depending on boundary conditions, in a further improvement.        statistical coherence is used in order to allow the user to draw frequency-dependent conclusions as to the usability or validity of the measurement.        it is possible to explicitly filter out frequency ranges by letting the user specify the filter function to be used. The time response filtered in this way may then be subjected to further investigations.        it is possible to perform a frequency analysis by windowing of the transfer function in the time domain, i.e. of the impulse response. Although this increases the signal-to-noise-ratio by excluding earlier or later portions in terms of time, it simultaneously prevents, depending upon the process used, the acquisition of a full system response.        it is possible, in a separate measurement, to determine a mean noise level, which allows the user to estimate the signal-to-noise-ratio present during the actual measurement, thus supporting any decision on taking further measures.        
None of these methods refer to the use of signals which are limited with regard to time and frequency and which, at the same time, vary both with regard to time and frequency, such as speech and music. In contrast to the automated approach the above methods are to be understood merely as interactive tools for the user during measuring. Interpretation and correct course of action are still left to a large extent to the user.
Measuring processes typically pursue one of the two following aims: (i) determination of the transfer function while exciting the system using the measuring apparatus itself, (ii) determination of the original signal input into a system through the approximate removal of changes by the system from the output signal.
In document DE 2 313 141 a process and an arrangement for averaging the transfer functions of systems in real time have been disclosed. With the known process for averaging the transfer functions of systems in real time provision is made for simultaneously forming the Fourier transforms of the input and output data and dividing these two transforms by each other.
In document DE 10 2006 004 105 A1 a device and a process for processing measured values has been disclosed. A measured value transducer is used for translating measured values into output signals.